1. Field of The Invention
The present invention relates generally to a method for producing a nickel-hydrogen cell.
2. Description of Related Background Art
In the positive electrode of a typical alkaline secondary cell, Ni(OH)2 is used as a reaction active material, and Co, CoO, Co(OH)2 and so forth are used as conductive materials. In a cell can, the metallic cobalt and cobalt compound contained in the positive electrode react with hydroxide ions, which are contained in an electrolyte, to form water soluble cobalt complex ions HCoO2xe2x88x92(CoOOHxe2x88x92) to diffuse. By charging the positive electrode in this state, high conductive CoOOH is deposited on the surface of Ni(OH2) of the positive electrode to form a network of a so-called cobalt conductive matrix (a conductive network), so that it is possible to improve the coefficient of use of the active material.
However, if the above described process for forming the conductive network is carried out in the cell can after assembling the cell, charge is started on the condition that the dissolved amount of HCoO2xe2x88x92 is small since the amount of the electrolyte in the cell can is small. For that reason, CoOOH having a bad solubility is deposited on Co(OH)2 and so forth, which should naturally dissolve in the electrolyte (if the amount of the electrolyte is sufficient), so that there is a problem in that it is not possible to sufficiently use the conductive material, such as Co(OH)2.
In addition, if the process for forming the conductive network in the cell can, the same quantity of electricity as that charged to Co, which does not serve as the active material of the positive electrode for practical use, is charged to the negative electrode, so that the same quantity of discharge reserve capacity at that charged to the conductive material of the positive electrode is formed when the formation of the cell is completed. Therefore, the used region of the negative electrode increases by the increase of the discharge reserve capacity of the negative electrode, so that the charge reserve capacity of the negative electrode decreases. For that reason, there are problems in that the number of cycles decreases until the charge reserve capacity of the negative electrode is lost due to the deterioration of the negative electrode and that the internal pressure rises at the last stage of charge when the negative electrode is deteriorated. In order to solve these problems, it is considered to adopt, for example, a technique for decreasing the amount of the conductive material or a technique for enhancing the proportion of Co(OH)2 that the quantity of electricity required to change to CoOOH is small. However, these techniques can cope with both of the formation of a good conductive network and the suppression of the used region of the negative electrode.
In addition, as a technique for forming a conductive network, there is also known a method for soaking Ni(OH)2 powder in a sufficient amount of alkaline solution, in which metallic cobalt or a cobalt salt is dissolved, before a paste is produced, to deposit CoOOH in this state. This method can solve the problem of the dissolved amount of HCoO2xe2x88x92. However, there is a problem in that the conductivity between Ni(HO)2 powders and between Ni(OH)2 powder and a collector is lowered after filling the collector with a paste of an active material to produce an electrode.
It is therefore an object of the present invention to eliminate the aforementioned problems and to provide a method for ensuring a right used region of a negative electrode and for forming a good cobalt conductive network in a positive electrode.
In order to accomplish the aforementioned and other objects, according to one aspect of the present invention, there is provided a method for producing a nickel-hydrogen cell, wherein a paste type positive electrode containing Ni(OH)2 as a reaction active material is charged to deposit CoOOH on the surface of Ni(OH)2 before the assembly of a cell is completed after the reaction active material is held by a collector.
That is, according to the present invention, it is possible to form the optimum conductive network by pre-charging a positive electrode while soaking the positive electrode in a sufficient amount of electrolyte before assembling a cell (before filling the positive electrode, together with a negative electrode and the, electrolyte, in a cell can) after pasting a reactions active material Ni(OH)2 to fill or apply the reaction active material in or on a collectors to cause the collector to hold the reaction active material (after forming the electrodes). In addition, since CoOOH is deposited on the electrode serving as thee final form, it is possible to obtain a stronger mechanical binding capacity and a higher conductivity than those of conventional electrodes.
In addition, if pre-charge is carried out according to the present invention, unlike if a cell is assembled without carrying out pre-charge, a charge of electricity corresponding to a charge of electricity given to Co, CoO and Co(OH)2 is not given to a negative electrode when the formation is completed, so that it is possible to control the used region of the negative electrode so that the used region is a right range.
The pre-charge conditions include the charge of the capacity of the positive electrode at a constant current of 0.05 CA to 0.2 CA ({fraction (1/20)} to ⅕ CA) or at a constant, voltage according thereto. If the current is too high, the formation of CoOOH is not enough, so that Ni(OH)2 is charged or gas is produced (the efficiency of the formation of CoOOH is lowered), and if the current is too low, the productivity is lowered. In view of them, the optimum current is about 0.1 CA ({fraction (1/10)} CA).
The pre-charge may be carried out by any one of the following two techniques:
(1) A technique for mixing at least one of Co, CoO and Co(OH)2 powders, Ni (OH)2 powder, and an additive, such as a suitable binder, to prepare a paste, filling or applying the paste in or on a collector to form a positive electrode, and then, soaking the positive electrode in a vessel, in which an electrolyte is housed, to charge the positive electrode; and
(2) A technique for mixing Ni(OH)2 powder with an additive, such as a suitable binder, to prepare a paste, filling or applying the paste in or on a collector to form a positive electrode, and then, soaking the positive electrode in a vessel housing therein an electrolyte, in which a cobalt compound, such as metallic cobalt or a cobalt salt, is dissolved therein, to charge the positive electrode.
Furthermore, the present invention can be applied regardless of the kind of the negative electrode. That is, the negative electrode may be made of any one of rare earth or Laves hydrogen absorbing alloys.